Steel plate having excellent pwht resistance for low-temperature pressure vessel and method for manufacturing same

ABSTRACT

The present disclosure relates to a steel plate having excellent PWHT resistance and low-temperature toughness for a low-temperature pressure vessel and a method for manufacturing the same, wherein the steel plate comprises, in terms of wt %, 0.07-0.17% of C, 0.15-0.40% of Si, 0.3-0.7% of Mn, 0.012% or less of P, 0.015% or less of S, 3.0-4.0% of Ni, 0.03-0.25% of W, and a balance of Fe and inevitable impurities, has a micro-structure comprising 25-80 area % of tempered bainite and the balance of tempered martensite, and has a tensile strength of 600 MPa or more.

TECHNICAL FIELD

The present disclosure relates to a thick steel plate used in alow-temperature pressure vessel, a ship, a storage tank, a structuralsteel, and the like, and a method for manufacturing the same, and moreparticularly, to a steel plate having excellent PWHT resistance andlow-temperature toughness for a low-temperature pressure vessel in whicha tensile strength is 600 MPa or more, and a method for manufacturingthe same.

BACKGROUND ART

As a thick plate steel material having high strength for low-temperatureuses, a mixed structure including ferrite, a martensite structure, and abainite structure, or a nearly single-phase structure mainly consistingof bainite and ferrite, or the like, is widely known.

It is necessary to have high strength, since the steel material itselfis usable as a structural material at the time of construction.Meanwhile, this high strength structural steel material is required tohave excellent PWHT resistance.

The high strength hot rolled steel material manufactured through ageneral normalizing treatment may have a mixed structure of ferrite andpearlite. However, when a PWHT treatment is performed in a subsequentprocess of a steel material having the structure, a carbide is formedalong a grain boundary, and thus, strength and toughness of the steelmaterial are lowered, failing to guarantee physical properties requiredfor PWHT. An example of the conventional technique regarding this isdisclosed in Korean Patent Laid-Open Publication No. 2012-0011289.

The Korean Patent Laid-Open Publication suggests a high strength steelmaterial for LPG having a strength of 500 MPa or more including: interms of wt %, 0.08-0.15% of C, 0.2-0.3% of Si, 0.5-1.2% of Mn,0.01-0.02% of P, 0.004-0.006% of S, more than 0%-0.01% or less of Ti,0.05-0.1% of Mo, 3.0-5.0% of Ni, and a balance of Fe and inevitableimpurities, characterized in that Ni and Mo are added to the steelcomposition component.

However, since the invention disclosed in the above-describedpublication is a steel material manufactured through generalnormalizing, there is a problem in that deterioration of strength andtoughness of the steel material after PWHT treatment is not avoidableeven in the case that Ni, or the like, is added.

Therefore, in a thick steel plate having high strength used in alow-temperature pressure vessel, a ship, a storage tank, a structuralsteel, and the like, there is growing demand for development of a highstrength steel material having excellent PWHT resistance for an extendedperiod of time.

RELATED ART DOCUMENT

(Patent Document 1) Korean Patent Laid-Open Publication No. 2012-0011289

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide a steel plate havingexcellent PWHT resistance for an extended period of time and highstrength of a low-temperature pressure vessel by controlling a steelcomposition, and cooling and heat treatment processes to form amicrostructure into a mixed structure of tempered bainite and temperedmartensite, and a method for manufacturing the same.

However, technical problems to be achieved in the present disclosure arenot limited to the above mentioned problems, and other non-mentionedproblems will be clearly understood by those skilled in the art from thefollowing descriptions.

Technical Solution

According to an aspect of the present disclosure, a steel plate havingexcellent PWHT resistance for a low-temperature pressure vesselincludes: in terms of wt %, 0.07-0.17% of C, 0.15-0.40% of Si, 0.3-0.7%of Mn, 0.012% or less of P, 0.015% or less of S, 3.0-4.0% of Ni,0.03-0.25% of W, and a balance of Fe and inevitable impurities, whereinthe steel plate has a microstructure including 25-80 area % of temperedbainite and a balance of tempered martensite.

The steel plate may maintain tensile strength at 600 MPa or more evenwhen the steel plate is subjected to PWHT for a maximum of 20 hours at580-640° C.

The steel plate may have a Charpy impact energy value of 200 J or moreat −110° C. even when the steel plate is subjected to PWHT for a maximumof 20 hours at 580-640° C.

According to another aspect of the present disclosure, a method formanufacturing a steel plate having excellent PWHT resistance for alow-temperature pressure vessel includes:

reheating a steel slab at a temperature of 1050-1250° C., the steel slabincluding, in terms of wt %, 0.07-0.17% of C, 0.15-0.40% of Si, 0.3-0.7%of Mn, 0.012% or less of P, 0.015% or less of S, 3.0-4.0% of Ni,0.03-0.25% of W, and a balance of Fe and inevitable impurities;

hot rolling the reheated steel slab and terminating the rolling at atemperature of 800° C. or more to obtain a hot rolled steel plate;

heating the hot rolled steel plate to 800-950° C., followed by watercooling at a cooling ate of 2.5-30° C./sec; and

tempering the water cooled steel plate at 550-660° C. for{1.5×t+(10−30)} minutes [wherein t is a thickness (mm) of the steelmaterial].

The method may further include: a PWHT process for a maximum of 20 hoursat 580-640° C. after the tempering.

A steel microstructure obtained by the tempering may have 25-80 areafraction % of tempered bainite and a balance of tempered martensite.

Advantageous Effects

As set forth above, according to an exemplary embodiment in the presentdisclosure, a steel plate having excellent PWHT resistance for alow-temperature pressure vessel, stably usable at a low temperature ofabout −110° C. while satisfying a tensile strength of 600 MPa or more,may be effective provided.

BEST MODE FOR INVENTION

Hereinafter, the present disclosure will be described in detail.

First, a steel plate having excellent PWHT resistance for alow-temperature pressure vessel of the present disclosure is described.

The steel plate of the present disclosure includes, in terms of wt %,0.07-0.17% of C, 0.15-0.40% of Si, 0.3-0.7% of Mn, 0.012% or less of P,0.015% or less of S, 3.0-4.0% of Ni, 0.03-0.25% of W, and a balance ofFe and inevitable impurities, and specific steel composition componentsand reasons for limiting the components are as follows.

In the present disclosure, C is preferably limited to a content of0.07-0.17%. When the content is less than 0.07%, a self-strength of amatrix may be deteriorated. When the content is more than 0.17%,weldability of the steel plate may be greatly deteriorated.

Si is a component to be added for a deoxidation effect, a solid-solutionstrengthening effect and an impact transition temperature increasingeffect, and is preferably added in a content of 0.15% or more in orderto achieve these additive effects. However, when the content of Si ismore than 0.40%, the weldability is lowered and an oxide film isexcessively formed on a surface of the steel plate, such that thecontent thereof is preferably limited to 0.15-0.40%.

Mn forms MnS, anon-metallic inclusion elongated together with S, tolower a room temperature elongation and a low-temperature toughness,such that a content of Mn is preferably controlled to be 0.7% or less.However, due to characteristics of the components of the presentdisclosure, when the content of Mn is less than 0.3%, it is difficult toensure adequate strength, such that an added content of Mn is preferablylimited to 0.3-0.7%.

P is an element that deteriorates low-temperature toughness, such that acontent of P is preferably suppressed as much as possible. However,since excessive cost is required to remove P in a steelmaking process,the content of P may be controlled within a range of 0.012% or less.

S is also an element that adversely affects low-temperature toughnesstogether with P, but as similar to P, excessive cost may be required toremove S in a steelmaking process, such that the content of S may becontrolled within a range of 0.015% or less.

Ni is the most effective element for improving low-temperaturetoughness. However, when an added content of Ni is less than 3.0%,low-temperature toughness may be deteriorated. When the added content ismore than 4.0%, production cost may be increased. Therefore, Ni ispreferably added within a range of 3.0 to 4.0%.

In the present disclosure, W is an important element that issolid-solutionized in austenite to increase curing ability of austeniteand precipitates into a carbide (W₂C) matched with a matrix to increasea strength of the steel. When W is added in a content of less than0.03%, an addition effect may not be expected. When W is added in acontent of more than 0.25%, a coarse precipitate may be formed during acasting process to deteriorate low-temperature toughness, such that thecontent of W is preferably limited to 0.03-0.25%.

Meanwhile, the steel plate of the present disclosure may have amicrostructure including 25-80 area % of tempered bainite and a balanceof tempered martensite. When a fraction of the tempered bainite is lessthan 25%, an amount of the tempered martensite may be excessive, andlow-temperature toughness of the steel plate may be deteriorated. On theother hand, when the fraction of the tempered bainite is more than 80%,it may be difficult to secure desired strength of the steel plate.

More preferably, the microstructure may include 30-70 area fraction % oftempered bainite and a balance of tempered martensite.

The steel plate having the above-described steel composition componentsand microstructure may effectively maintain tensile strength at 600 MPaor more and have excellent low-temperature toughness even when the steelplate is subjected to PWHT for a maximum of 20 hours at 580-640° C.

Next, a method for manufacturing a steel plate having excellent PWHTresistance for a low-temperature pressure vessel of the presentdisclosure will be described.

The method for the steel plate of the present disclosure includes:reheating a steel slab having the above-described steel compositioncomponents at 1050-1250° C.; hot rolling the reheated steel slab andterminating the rolling at a temperature of 800° C. or more to obtain ahot rolled steel plate; heating the hot rolled steel plate to 800-950°C., followed by water cooling at a cooling rate of 2.5-30° C./sec; andtempering the water cooled steel material at 550-660° C. for{1.5×t+(10−30)} minutes [wherein t is a thickness (mm) of the steelmaterial].

First, in the present disclosure, the steel slab having the steelcomposition component is reheated at 1050-1250° C. When a reheatingtemperature is less than 1050° C., a solute atom may be difficult to besolid-solutionized, and when the reheating temperature is more than1250° C., an austenite crystal grain size may be excessively coarse tolower physical properties of the steel plate.

Subsequently, in the present disclosure, the reheated steel slab may behot rolled. Specifically, in the present disclosure, the reheated steelslab may be hot rolled, and the rolling may be terminated at atemperature of 800° C. or more. When a hot rolling temperature is lessthan 800° C., hot deformation resistance may increase at the time ofrolling, which may result in a load on a rolling mill.

A reduction rate per pass in the hot rolling is preferably 5-30%.

In addition, in the present disclosure, the hot rolled steel plate maybe heated at 800-950° C. and then water cooled at a cooling rate of2.5-30° C./sec.

When a heating temperature is less than 800° C., an alloy component maybe difficult to be sufficiently solid-solutionized, and when the heatingtemperature is more than 950° C., crystal grains may be coarsened, suchthat the toughness may be deteriorated.

In addition, when the cooling rate is less than 2.5° C./sec, themartensite structure may not be obtained. On the other hand, when thecooling rate is more than 30° C./sec, since a large amount of coolingwater may be required, there is an economic burden of requiringadditional cooling equipment, such that the cooling rate is preferablylimited to 2.5-30° C./sec.

Subsequently, in the present disclosure, the water cooled steel platemay be tempered.

Specifically, in the present disclosure, the water cooled steel platemay be tempered at 550-660° C. for {1.5×t+(10−30)} minutes [wherein t isa thickness (mm) of the steel material]. When a tempering temperature isless than 550° C., the toughness may be deteriorated by excessivestrength. When the tempering temperature is more than 660° C., strengthmay be excessively deteriorated.

Further, in the present disclosure, a tempering time may be determinedto {1.5×t+(10−30)} minutes [wherein t is a thickness (mm) of the steelmaterial], and a specific reason for the limitation is as follows.

In other words, when the tempering time is shorter than the abovecriteria, the tempered martensite structure may be difficult to beobtained. On the other hand, when the tempering time is longer than theabove criteria, overall productivity may be damaged.

By the tempering heat treatment under the above-described condition, thesteel microstructure including 25-80 area % of tempered bainite and abalance of tempered martensite may be obtained.

More preferably, the microstructure may include 30-70 area fraction % oftempered bainite and a balance of tempered martensite.

Subsequently, in the present disclosure, PWHT heat treatment may beperformed on the tempered steel plate to remove stress of a weldedportion after welding for manufacturing a pressure vessel. In otherwords, a PWHT process for a maximum of 20 hours at 580-640° C. may befurther included.

When a PWHT temperature is less than 580° C., it may be difficult toremove residual stress from the welded portion, or the like, and whenthe PWHT temperature is more than 640° C., strength of the steelmaterial may be significantly lowered. Further, when the PWHT time ismore than 20 hours, strength may be excessively deteriorated.

MODE FOR INVENTION

Hereinafter, the present disclosure will be described in more detailthrough Examples.

Steel slabs having composition components shown in Table 1 below wereprepared, respectively, and these steel slabs were reheated at 1100° C.Then, the reheated steel slabs were hot rolled at a reduction rate of15% per pass, and the hot rolling was terminated at 900° C. tomanufacture hot rolled steel plates having a predetermined thickness.

The hot rolled steel plates were heated at an austenitizationtemperature and water cooled under conditions shown in Table 2 below,and subsequently, tempered at temperature and time shown in Table 2below. In addition, the tempered steel plates were also subjected toPWHT treatment under the conditions shown in Table 2 below.

As described above, the PWHT-treated steel plates were evaluated foryield strength, tensile strength and low temperature toughness, andresults thereof are also shown in Table 2 below. Meanwhile, in Table 2below, the low temperature toughness is a result of evaluating aspecimen having V notch at −110° C. with the Charpy impact energy valueobtained by performing the Charpy impact test.

TABLE 1 Kind of Composition component (wt %) steel C Mn Si P S Ni WInventive 0.10 0.62 0.29 0.009 0.0012 3.49 0.08 steel a Inventive 0.090.60 0.27 0.008 0.0010 3.45 0.11 steel b Inventive 0.10 0.65 0.28 0.0100.0011 3.55 0.18 steel c Comparative 0.11 0.68 0.29 0.012 0.0012 3.50 —steel d

TABLE 2 Temper- Tempered Heating Water ing Temper- PWHT bainite −110° C.Kind temper- cooling temper- ing temper- PWHT area Impact of ature rateature time ature time fraction YS TS toughness Classification steel (°C.) (° C./s) (° C.) (min) (° C.) (hr) (%) (Mpa) (Mpa) (J) Inventive a850 15.0 650 50 630 15 60 568 608 256 Example 1 Inventive 860 8.5 650 90630 20 55 557 602 251 Example 2 Inventive b 850 15.0 650 50 630 15 53558 610 227 Example 3 Inventive 860 8.5 650 90 630 20 50 557 605 233Example 4 Inventive c 850 15.0 650 50 630 15 48 560 615 230 Example 5Inventive 850 8.5 650 90 630 20 45 551 610 215 Example 6 Comparative D850 Air 650 50 630 15 0 458 523 155 Example 1 cooling Comparative 850Air 650 90 630 20 0 442 516 148 Example 2 cooling

As shown in Tables 1 and 2, it could be appreciated that in InventiveExamples 1-6 in which the steel composition components and themanufacturing process conditions satisfy the range of the presentdisclosure, after tempering treatment, the structure including 25-80area fraction % of tempered bainite and a balance of tempered martensitecould be obtained, such that as compared to Comparative Examples, yieldstrength and tensile strength after the subsequent PWHT were higher byabout 100 MPa and 80 MPa, respectively, and −110° C. low temperaturetoughness was also higher by 70 J or more.

On the other hand, since the comparative steel d did not contain W,strength of the steel was relatively low. In Comparative Examples 1 and2, since water cooling was not performed but air cooling was performed,the tempered bainite was not generated, such that the yield strength andtensile strength after the subsequent PWHT were lower than those of theInventive Example, and the −110° C. low temperature toughness was alsolower than that of the Inventive Example.

1. A steel plate having excellent PWHT resistance for a low-temperaturepressure vessel comprising: in terms of wt %, 0.07-0.17% of C,0.15-0.40% of Si, 0.3-0.7% of Mn, 0.012% or less of P, 0.015% or less ofS, 3.0-4.0% of Ni, 0.03-0.25% of W, and a balance of Fe and inevitableimpurities, wherein the steel plate has a steel microstructure including25-80 area % of tempered bainite and a balance of tempered martensite.2. The steel plate of claim 1, wherein the steel plate maintains tensilestrength at 600 MPa or more even when the steel plate is subjected toPWHT for a maximum of 20 hours at 580-640° C.
 3. The steel plate ofclaim 1, wherein the steel microstructure includes 30-70 area % oftempered bainite and a balance of tempered martensite.
 4. The steelplate of claim 1, wherein the steel plate has a Charpy impact energyvalue of 200 J or more at −110° C. even when the steel plate issubjected to PWHT for a maximum of 20 hours at 580-640° C.
 5. A methodfor manufacturing a steel plate having excellent PWHT resistance for alow-temperature pressure vessel, the method comprising: reheating asteel slab at a temperature of 1050-1250° C., the steel slab including,in terms of wt %, 0.07-0.17% of C, 0.15-0.40% of Si, 0.3-0.7% of Mn,0.012% or less of P, 0.015% or less of S, 3.0-4.0% of Ni, 0.03-0.25% ofW, and a balance of Fe and inevitable impurities; hot rolling thereheated steel slab and terminating the rolling at a temperature of 800°C. or more to obtain a hot rolled steel plate; heating the hot rolledsteel plate to 800-950° C., followed by water cooling; and tempering thewater cooled steel material at 550-660° C. for {1.5×t+(10−30)} minutes[wherein t is a thickness (mm) of the steel material].
 6. The method ofclaim 5, further comprising: a PWHT process for a maximum of 20 hours at580-640° C. after the tempering.
 7. The method of claim 5, wherein asteel microstructure obtained by the tempering has 25-80 area % oftempered bainite and a balance of tempered martensite.
 8. The method ofclaim 5, wherein a steel microstructure obtained by the tempering has30-70 area % of tempered bainite and a balance of tempered martensite.9. The method of claim 5, wherein a reduction rate per pass in the hotrolling is 5-30%.
 10. The method of claim 5, wherein a cooling rate inthe water cooling is 2.5 to 30° C./sec.